This invention relates to flowmeters, and more particularly relates to flowmeters using doppler principles for their operation.
The use of doppler principles for flowmeters is well known. In one well-known system, which is commercially used, a transmitter transducer crystal and receiver transducer crystal are contained in a common housing which is connected to a conduit which carries fluid. The transmitter crystal produces a continuous wave output at a single stable high frequency. The continuous wave sonic signal is injected through the wall of the conduit and into the fluid. If the fluid being monitored contains particles having a size greater than about 1/10th the wavelength of the sonic frequency used to make the measurement, a sonic wave will be reflected from the particles and received by the receiver transducer crystal. If the particle is moving relative to the receiver transducer, the frequency of the received wave will be doppler-shifted from the frequency of the transmitted wave. The doppler shift is proportional to the velocity of the reflecting particle and thus to the velocity of the fluid. This velocity is then indicated.
The single transducer system described above has a shortcoming in that the transmit signal applied to the receiver generally predominates over the particle-reflected signal. The beat frequency amplitude modulation then becomes too low to be reliably measured. This loss of modulation is especially prevalent in cases of low reflected signal amplitude.
In an attempt to avoid the problems inherent in the single transducer housing arrangement, the transmit and receive transducers have been separately housed and mounted at spaced locations on the conduit containing the fluid to be monitored. Since the receive transducer must receive some of the transmit signal in order to produce the beat frequency, the transmit signal was derived directly from the conduit wall. However, since the transducers are physically separated, it is possible to adjust the ratio of transmit signal to receive signal received by the receive transducer to keep their relative amplitudes equalized. The transmit signal and receive signal are then added in the receive transducer in the usual manner to produce an amplitude-modulated beat frequency which is demodulated to produce an indication of flow.
In using the above systems, the devices will frequently work well for a period of time and then unexpectedly will work poorly for a period of time. These problems are frequently attributed to mechanical defects or circuit defects.